Line pipes, which are used for transporting natural gas, crude oil, and the like, have been strongly required to have higher strength in order to improve transport efficiency by using higher pressure and improve on-site welding efficiency by using pipes with thinner walls. In particular, line pipes for transporting high-pressure gas (hereinafter also referred to as high-pressure gas line pipes) are required to have not only material properties such as strength and toughness, which are necessary for general-purpose structural steel, but also material properties related to fracture resistance, which are specific to gas line pipes.
Fracture toughness values of general-purpose structural steel indicate resistance to brittle fracture and are used as indices for making designs so as not to cause brittle fracture during use. For high-pressure gas line pipes, prevention of brittle fracture alone for avoiding catastrophic fracture is not sufficient, and prevention of ductile fracture called unstable ductile fracture is also necessary.
The unstable ductile fracture is a phenomenon where a ductile fracture propagates in a high-pressure gas line pipe in the axial direction of the pipe at a speed of 100 m/s or higher, and this phenomenon can cause catastrophic fracture across several kilometers. Thus, a Charpy impact absorbed energy value and a DWTT (Drop Weight Tear Test) value necessary for preventing unstable ductile fracture are determined from results of past gas burst tests of full-scale pipes, and high Charpy impact absorbed energies and excellent DWTT properties have been demanded. The DWTT value as used herein refers to a fracture appearance transition temperature at which a percent ductile fracture is 85%.
In response to such a demand, Patent Literature 1 discloses a steel plate for steel pipes that has a composition that forms less ferrite in a natural cooling process after rolling, and a method for producing the steel plate. By performing the rolling at an accumulated rolling reduction ratio at 700° C. or lower of 30% or more, the steel plate has a microstructure including a developed texture and composed mainly of bainite, and the area fraction of ferrite present in prior-austenite grain boundaries is 5% or less, so that the steel plate is provided with a high Charpy impact absorbed energy and excellent DWTT properties.
Patent Literature 2 discloses a method for producing a high-strength steel plate having a thickness of 15 mm or less. By rolling a steel containing, by mass %, C: 0.03% to 0.1%, Mn: 1.0% to 2.0%, Nb: 0.01% to 0.1%, P≤0.01%, S≤0.003%, and O≤0.005% in a temperature range from (Ar3+80° C.) to 950° C. at an accumulated rolling reduction ratio of 50% or more, performing natural cooling for a while, and performing rolling in a temperature range from Ar3 to (Ar3−30° C.) at an accumulated rolling reduction ratio of 10% to 30%, the steel plate has an undeveloped rolling texture and deformed ferrite, undergoes no separation, and has a high absorbed energy.
Patent Literature 3 discloses a high-tensile steel plate and a method for producing the steel plate. By subjecting a steel containing, by mass %, C: 0.02% to 0.1%, Si: 0.6% or less, Mn: 1.6% to 2.5%, Ni: 0.1% to 0.7%, Nb: 0.01% to 0.1%, and Ti: 0.005% to 0.03% and having a carbon equivalent Pcm of 0.180% to 0.220% to predetermined continuous casting while suppressing the center segregation of Mn, performing hot rolling under predetermined conditions, performing cooling from a temperature equal to or higher than (Ar3−50° C.) to a temperature range of 300° C. to 500° C. at a cooling rate of 10° C./s to 45° C./s, and optionally performing tempering at a temperature lower than Act temperature, the Martensite-Austenite constituent fraction and hardness of a surface portion are reduced, and the steel plate is provided with high toughness and excellent high-speed ductile fracture properties.
Patent Literature 4 discloses a high-strength, high-toughness steel plate including bainite or martensite, wherein cementite present in the bainite or martensite has an average particle size of 0.5 μm or less. By hot-rolling a steel containing, by mass %, C: 0.03% to 0.12%, Si≤0.5%, Mn: 1.5% to 3.0%, Nb: 0.01% to 0.08%, Ti: 0.005% to 0.025%, and at least one of Cu, Ni, Cr, Mo, V, and B at an accumulated rolling reduction ratio ≥67% in an austenite non-recrystallization temperature range of 950° C. or lower, performing cooling from a cooling start temperature of 600° C. or higher to a temperature range of 250° C. or lower at a cooling rate of 20° C./s to 80° C./s, and performing reheating to 300° C. to 500° C., the steel plate is provided with high resistance to crack by cutting and excellent DWTT properties.